Method of causing intergranular stress corrosion crack to generate and grow in sample

ABSTRACT

To provide a method for simply and easily causing only IGSCC to initiate and grow in a specimen, which can retain corrosion resistance through passivation, under atmospheric pressure in a short time, an aqueous solution of potassium tetrathionate is brought into contact with the specimen capable of retaining corrosion resistance through passivation. The potassium tetrathionate concentration, temperature and pH value of the aqueous solution are from 0.3 to 6 wt. %, from 5 to 60° C., and from 3 to 6, respectively.

TECHNICAL FIELD

This invention relates to a maintenance technique for intergranularstress corrosion cracking (hereinafter abbreviated as “IGSCC”) known asenvironmentally-assisted cracking in a weld of a structure, for example,in a nuclear power plant, and especially to a technique which can causeIGSCC to initiate and grow by a simple and easy method in a specimencapable of retaining corrosion resistance through passivation.

BACKGROUND ART

Under the current circumstances, the construction of new nuclear powerplants is difficult although there is an increasing worldwiderequirement for successfully dealing with both a reduction in CO₂emission and an increase in electricity demand. For the extension of theoperational life of each existing nuclear power plant operated overalong period, it is therefore important to determine the soundness ofmaterials of its equipment and the like such as its reactor and in-corestructures and weld zones therein.

As a method for non-destructively detecting and determining theexistence or non-existence of cracks formed through IGSCC in thematerials of equipment such as a reactor due to conditions such asquality, stress and use environment as well as their dimensions, shapesand the like in the actual equipment, it can be contemplated to applyultrasonic examination technology, eddy current examination technologyor the like.

For the extensive application of the detection and determination ofcracks by the above-described technology to positions exposed to variousconditions, however, it is necessary to repeat the detection anddetermination of cracks by using specimens with IGSCC, which occur undervarious conditions in actual equipment, reproduced experimentallytherein and to accumulate data.

Under conditions where a corrosion flaw other than IGSCC to bedetermined exists in combination, it is difficult to accurately detectand determine a crack formed by IGSCC. There is, accordingly, anoutstanding demand for the development of a technique that canexperimentally cause only IGSCC to initiate and grow in a short timeexclusively at an intended specific area although in actual equipment,IGSCC occurs after an extremely long time under conditions ofhigh-temperature and high-pressure water.

FIGS. 2A and 2B are construction diagrams of a shroud support, which isone of the in-core structures in a pressure vessel in a nuclear powerplant, and FIG. 2B is an enlarged view of a section X in FIG. 2A. Thesefigures show the pressure vessel at numeral 100, legs 101 each arrangedupright on a bottom part of the pressure vessel 100 via a weld 102, anda support cylinder 103 supported on the legs 101. The support cylinderis connected to each leg 101 via a weld 104. Designated at numeral 105are support plates arranged between the pressure vessel 100 and thesupport cylinder 103 and connected to the pressure vessel 100 and thesupport cylinder 103 via welds 106.

Widely employed as the material of this structure is a nickel-basedalloy or stainless steel which can retain corrosion resistance throughpassivation. With such a material, the application of stress to asensitized area in the vicinity of grain boundaries (i.e., Cr-depletedarea as a result of the deposition of chromium carbides) results in theinitiation of a crack, followed by its growth. This is called “IGSCC”.

As a method for causing IGSCC to initiate and grow in a partial specimenof a welded structure in equipment, said welded structure being capableof retaining corrosion resistance through passivation, an acceleratedtest has been widely used to date. This accelerated test is conductedusing as a starting point a simulated defect, which has been formedartificially by cutting or electrical discharge machining, as is or adefect of weld crack, and takes a long time in high-temperature andhigh-pressure water.

Although not studied for the above purpose, there is ASTM G35 test[Wachenroder's solution, pH<1], which makes use of polythionic acid andmay be used as a reference for the simple and easy initiation and growthof IGSCC. This method is known in connection with studies on IGSCC ofsensitized stainless steel in desulfurization equipment in the petroleumrefinery industry [see Matsushima et al: Boshoku Gijutsu (CorrosionPreventive Technology), 22(4), (1974)].

However, the above-mentioned test simulates the environment ofdesulfurization equipment. It has, therefore, been reported that thepreparation of the test solution is not simple and easy and also thatnot only IGSCC but also an intergranular corrosion (IGC) attack occurs(see the non-patent publication referred to in the above). Therefore,cracking occurred under test conditions, under which IGC was observed,is considered to be a stress-accelerated IGC phenomenon rather thanIGSCC.

As is appreciated from the foregoing, no simple and easy technique hasheretofore been established for the development of only IGSCC withoutIGC or pitting corrosion at room temperature in the atmosphere exceptfor the method that can cause IGSCC to initiate and grow by relying upona long-time accelerated test in high-temperature and high-pressurewater.

An object of the present invention is, therefore, to overcome theabove-described drawbacks of the conventional techniques, and to providea method for simply and easily causing only IGSCC to initiate and growin a specimen, which can retain corrosion resistance throughpassivation, in a short time in the atmosphere.

DISCLOSURE OF THE INVENTION

To achieve the above-described object, a first aspect of the presentinvention is characterized by bringing a specimen, which is made of amaterial capable of retaining corrosion resistance through passivation,into contact with a solution of a tetrathionate salt such thatintergranular stress corrosion cracking initiates and grows in thespecimen.

A second aspect of the present invention is characterized in that thespecimen is a welded structure or a simulated specimen of the weldedstructure, or a cut-out specimen cut out from the welded structure orthe simulated specimen.

A third aspect of the present invention is characterized by providingmeans for applying a strain to the specimen such that the specimen isbrought into contact with the tetrathionate salt solution while beingapplied with a strain by the means.

A fourth aspect of the present invention is characterized in that themeans for applying a strain to the specimen is a weld applied to thespecimen.

A fifth aspect of the present invention is characterized in that themeans for applying a strain to the specimen is a member or device forapplying a strain to the specimen from an outside.

A sixth aspect of the present invention is characterized in that thesolution of the tetrathionate salt is an aqueous solution of potassiumtetrathionate or sodium tetrathionate, and a concentrate of potassiumtetrathionate or sodium tetrathionate, a temperature of the solution anda pH value of the solution are controlled to a range of from 0.3 to 6wt. %, a range of from 5 to 60° C. and a range of from 3 to 6,respectively.

A seventh aspect of the present invention is characterized in that theaqueous solution of potassium tetrathionate or sodium tetrathionatecontains chlorine in a range of from 0.06 to 6 wt. %, for example, byaddition of NaCl or KCl.

An eighth aspect of the present invention is characterized in aconcentration of the chlorine is controlled to a range of from 0.6 to 6wt. %.

A ninth aspect of the present invention is characterized by covering thespecimen with a non-metallic material at an area other than an areawhere initiation of the intergranular stress corrosion cracking isintended.

A tenth aspect of the present invention is characterized in that thenon-metallic material is a silicone rubber or fluororubber.

An eleventh aspect of the present invention is characterized in that thespecimen has a weld, and the weld is composed of a nickel-base alloy orstainless steel.

A twelfth aspect of the present invention is characterized in that thespecimen contains C, Nb and Ti, and a stabilization parameter of amaterial making up the specimen as determined by the following formulais 12 or smaller:Stabilization parameter=0.13×[(Nb+2Ti)/C]where C, Nb and Ti represent contents of the respective constituentmaterials as expressed in terms of wt. %.

A thirteenth aspect of the present invention is characterized byapplying heat treatment to the specimen to bring the specimen into asensitized state before causing the intergranular stress corrosioncracking to initiate in the specimen.

A fourteenth aspect of the present invention is characterized in thatthe heat treatment is applied to the specimen within the followingtemperature range:500° C.≦T° C.≦650° C.and the following time range:t₁ hours≦t hours≦t₂ hourswhere $\begin{matrix}{t_{1} = {6\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\{t_{2} = {15\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\{{Q = {50,000\quad{to}\quad 60,000\quad{cal}\text{/}{mol}}},{and}} \\{R = {1.987\quad{cal}\quad K^{- 1}{{mol}^{- 1}.}}}\end{matrix}$

A fifteenth aspect of the present invention is characterized in that thespecimen has a weld, and the weld is composed of a ferrite-containingaustenite stainless steel.

A sixteenth aspect of the present invention is characterized in that thespecimen has a weld, a surface of a weld bead of the weld is ground atan area thereof where initiation of intergranular stress corrosioncracking is intended, and the solution of the tetrathionate salt isbrought into contact with the thus-ground surface.

A seventeenth aspect of the present invention is characterized in that adam is formed on the specimen in a vicinity of an area where initiationof intergranular stress corrosion cracking is intended, and the solutionof the tetrathionate salt is poured into the dam to cause theintergranular stress corrosion cracking to initiate and grow in oraround the weld.

An eighteenth aspect of the present invention is characterized in thatthe specimen is a welded structure in a nuclear power plant or asimulated specimen of the welded structure, or a cut-out specimen cutout from the welded structure or the simulated specimen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic diagram showing a range of test conditions of thisinvention.

FIGS. 2A and 2B are views illustrating an in-core structure of apressure vessel in a nuclear power plant.

FIG. 3 is a table showing illustrative chemical compositions ofnickel-based alloy test pieces for use in embodiments of the presentinvention.

FIG. 4 is a table showing test conditions for the initiation and growthof IGSCC.

FIG. 5 shows perspective views of test pieces for use in embodiments ofthe present invention.

FIGS. 6A and 6B are views illustrating one of the test pieces in a statemounted on a jig.

FIG. 7 is a table showing test conditions for the initiation and growthof IGSCC and the test results.

FIG. 8 is a photograph showing the conditions of a test piece, to whicha stress strain had been applied, after the test piece was tested.

FIG. 9 is a photograph showing the conditions of cracking of a testpiece to which a stress strain had been applied.

FIG. 10 is a photograph showing the conditions of a test piece, to whicha stress strain had been applied, after the test piece was tested.

FIG. 11 shows photographs of the conditions of cracking of a test pieceto which a stress strain had been applied.

FIG. 12 is a photograph showing the conditions of cracking of a testpiece to which a stress strain had been applied.

FIG. 13 is a photograph showing the conditions of a test piece, to whicha stress strain had not been applied, after the test piece was tested.

FIG. 14 is a photograph showing the conditions of a test piece, to whicha stress strain had not been applied, after the test piece was tested.

FIG. 15 is a photograph showing the conditions of a test piece, to whicha stress strain had not been applied, after the test piece was tested.

FIG. 16 is a conception diagram of effects of a load strain on theservice life until SCC initiation.

FIG. 17 is a table showing test conditions for the initiation and growthof IGSCC and the test results.

FIGS. 18A through 18D are tables showing test conditions for theinitiation and growth of IGSCC versus SN ratios.

FIG. 19 is a graphic diagram of the addition of NaCl versus SN ratio.

FIG. 20 is a graphic diagram showing effects of load strain on theservice life of Alloy 180 until SCC in test solutions.

FIG. 21 is a perspective view of a specimen.

FIG. 22 is a perspective view of a specimen in a form coated withsilicone rubber.

FIG. 23 is a table showing two kinds of test conditions and the testresults.

FIG. 24 is a view depicting an illustrative test arrangement.

FIG. 25 shows photographs, which present details of cracking in a testpiece.

FIG. 26 shows a photograph and a simplified view, which present detailsof cracking in a test piece.

FIG. 27 is a photograph showing conditions of a fracture surface of atest piece.

FIG. 28 shows perspective views of further specimens.

FIG. 29 is a perspective view of a still further specimen.

FIG. 30 is a view illustrating a yet further specimen under testing.

FIG. 31 is a view showing a still yet further specimen under testing.

FIG. 32 is a flow chart of a first testing method.

FIGS. 33A through 33D are perspective views, which specificallyillustrate the testing method making use of a grinding means.

FIG. 34 is a flow chart of a second testing method.

FIG. 35 is a flow chart of a third testing method.

FIG. 36 is a photograph of a specimen, which shows a grinder-finishedarea and grinder-unfinished area of a weld bead.

FIG. 37 is a photograph showing that patterns of IGSCC had been detectedon the specimen.

FIG. 38 is a simplified view showing an illustrative welded joint in anin-core structure of a pressure vessel.

FIG. 39 is a table showing an illustrative chemical composition of anaustenite stainless steel weld metal.

FIG. 40 is a table showing welding conditions upon preparing a specimen.

FIG. 41 is a diagram illustrating how to cut out the specimen.

FIG. 42 is a table showing the conditions of heat treatment for thespecimen.

FIG. 43 is a table showing the test conditions of the modified ASTMA262E method.

FIG. 44 is a simplified construction diagram showing the arrangement fora test by the modified ASTM A262E method.

FIG. 45 is a photograph showing a cross-section of the specimen afterthe test by the modified ASTM A262E method.

FIG. 46 is a graphic diagram of heating time versus intergranularcorrosion speed as determined by the modified ASTM A262E method.

FIG. 47 is a perspective view showing how to test a sensitized specimen.

FIG. 48 is a simplified view showing the state of IGSCC initiated andgrown in an austenite stainless steel weld metal.

FIG. 49 is a graphic diagram of the pH value of test solution versusS/N.

BEST MODES FOR CARRYING OUT THE INVENTION

With reference to the drawings, a description will next be made indetail about a method according to an embodiment of the presentinvention for the initiation and growth of IGSCC in a specimen.

FIG. 1 is a graphic diagram showing conditions for a test solution inthe present invention. As indicated by hatching in the figure, anaqueous solution of potassium tetrathionate (K₂S₄O₆) or sodiumtetrathionate (Na₂S₄O₆) having a concentration of from 0.3 to 6 wt. %and a pH value of from 3 to 6 (hereinafter to be mentioned about theaqueous solution of potassium tetrathionate as a representative example)is brought into contact with a specimen at a temperature of from 5 to60° C. under the present invention.

The specimen may be brought into contact with the aqueous solution ofpotassium tetrathionate under the above-described conditions while beingcovered, as needed, with a non-metallic material such as a siliconerubber of fluororubber at a metal surface other than an area where IGSCCis intended to grow in the test piece.

If needed to sensitize the specimen, the stabilization parameter of thematerial of at least a portion of the specimen can be controlled to 12or lower, or stress relief annealing can be applied to the test piece ataround 600° C., followed by application of low-temperature aging at 288to 550° C.

The stabilization parameter is determined by the following formula:Stabilization parameter=0.13×[(Nb+2Ti)/C]where C, Nb and Ti indicate the contents of the respective constituentmaterials as expressed in terms of wt. %.

FIG. 3 is a table showing the chemical compositions of Alloy 182 andAlloy 600 as examples of a nickel-base alloy usable as a specimen in thepresent invention. As shown in the table, Alloy 182 and Alloy 600 bothhave a Ni content higher than 70 wt. %.

The stabilization parameter of Alloy 182 is 7.3 or smaller, while thatof Alloy 600 is 1.5 or smaller. As their stabilization parameters areboth smaller than 12, specimens can be sensitized by stress reliefannealing at 600° C. and low-temperature aging at 288 to 550° C.

FIG. 4 shows the conditions (test solution designations: A to D) fortests conducted to establish a technique for the initiation and growthof IGSCC, and also, the conditions (test solution designation: E) for anIGSCC test in high-temperature water as estimated from technical papers.Under the invention conditions A shown in the table, the test isconducted at atmospheric pressure and room temperature (RT) with a 1 wt.% aqueous solution of (pH value: about 5) without adding any acid for pHadjustment.

On the other hand, the comparative conditions B are different from theinvention conditions A in that the temperature has been raised to 80° C.The comparative conditions C are different from the invention conditionsA in that the concentration of the aqueous solution of K₂S₄O₆ has beenraised to 10%. The comparative conditions D are the conditions for acrevice SCC test at a pressure of 8.3 MPa in high-temperature water(solution temperature: 288° C.). The comparative conditions E are SCCconditions simulating the environment in actual equipment, thesurrounding water is actual reactor water with 0.2 ppm of dissolvedoxygen contained therein, and the pressure and solution temperature werethe saturated vapor pressure and 288° C., respectively.

FIG. 5 shows perspective views of test pieces for use in tests under theinvention condition A, the comparative conditions B and the comparativeconditions C. The test piece A is a test piece of Alloy 182 weld metalfor IGSCC-initiating, fixed-strain bend test. The test piece B is a testpiece of sensitized Alloy 600 for investigating whether or not IGC orpitting corrosion has occurred or not.

In FIGS. 6A and 6B, the test piece A for fixed-strain bend test ismounted on a jig by bolts and nuts to develop a load (bend) strain to1.5%. The figures depict the bend-strain applying jig 1 with an uppersurface thereof bulging out in a slightly curved profile, the test piece2 set on the jig 1 under the bend strain applied thereto, a siliconerubber coating 3 covering the test piece 2 at an area other than acentral area such that the area where initiation of IGSCC is intendedcan be limited, a target IGSCC initiation area 4 over which the coating3 is not applied, test piece holding washers 5, bolts 6, and nuts 7. Bysetting the test piece 2 on the jig 1 as illustrated in FIG. 4, a fixedbend strain is loaded on the test piece 2 from the outside.

FIG. 7 is a table in which the results of those tests are summarized,and presents the test conditions (surrounding water, pressures, solutiontemperatures, load strains), testing time, and the results ofexaminations as to whether or not the cracking was IGSCC. In the table,Tests Nos. 1 to 6 were each studied as an alternative testing method ofthe test in high-temperature and high-pressure water of 288° C., saidtest requiring a long time to initiate IGSCC as demonstrated in TestsNos. 7 and 8. It is to be note that the value of the testing time inTest No. 8 was a value estimated from empirical data.

FIG. 8 shows a surface of the test piece after tested for 72 hours inTest No. 1 (1 wt. % aqueous solution of K₂S₄O₆, atmospheric pressure,solution temperature: room temperature, load strain: 1.5%) shown in FIG.7. According to examinations conducted in the course of the test, it wasconfirmed that microcracks (fine cracks) occurred at the 24^(th) hourand cracks were visually observed at the 48^(th) hour. Indicated by anarrow in FIG. 8 is a silicone rubber coating applied to the test pieceat an area where initiation of SCC was not intended. Initiation of IGSCCis observed at a central area in the figure.

FIG. 9 shows a surface of the test piece after tested for 120 hours inTest No. 2 (1 wt. % aqueous solution of K₂S₄O₆, atmospheric pressure,solution temperature: 80° C., load strain: 1.5%) shown in FIG. 7. IGSCCwas hard to initiate, and pitting corrosion was observed. As IGSCC ashard to initiate, the testing time took as much as 120 hours. Accordingto the photograph, pitting corrosion initiated on the surface of thetest piece without any pronounced large cracks. The photograph was takenafter removing the silicone rubber coating.

FIG. 10 shows a surface of the test piece after tested for 72 hours inTest No. 3 (10 wt. % aqueous solution of K₂S₄O₆, atmospheric pressure,solution temperature: room temperature, load strain: 1.5%) shown in FIG.7. It was confirmed in the course of the test that microcracks (finecracks) occurred at the 24^(th) hour and cracks (IGSCC+IGC) werevisually observed at the 48^(th) hour in a central part of thephotograph. Therefore, the cracking in each of Tests Nos. 2 and 3 wasnot true IGSCC but was a stress-accelerated IGC phenomenon.

FIG. 11 shows photographs of the conditions of cracking occurred inAlloy 182 as a result of Test No. 1 as viewed in a cross-section, andFIG. 12 is an enlarged photograph showing the details of a fracturesurface of the alloy. As shown in these photographs, the crackingoccurred along grain boundaries, and this cracking was confirmed to beIGSCC.

FIG. 13 shows a cross-section of the test piece in a slightly bent formafter conducting Test No. 4 (1 wt. % aqueous solution of K₂S₄O₆,atmospheric pressure, solution temperature: room temperature, loadstrain: 0%, testing time: 168 hours) in which the immersion of the longtime was conducted without applying any fixed-strain bend. As shown inthe photograph, the test piece was in a passivated state withoutobservation of any pitting corrosion or IGC.

FIG. 14 shows a cross-section of the test piece in a slightly bent formafter conducting Test No. 5 (1 wt. % aqueous solution of K₂O₄₀₆,atmospheric pressure, solution temperature: 80° C., load strain: 0%,testing time: 264 hours) in which the immersion of the long time wasconducted without applying any fixed-strain bend. As shown in thephotograph, pitting corrosion and IGC were observed.

FIG. 15 shows a cross-section of the test piece in a slightly bent formafter conducting Test No. 6 (10 wt. % aqueous solution of K₂S₄O₆,atmospheric pressure, solution temperature: room temperature, loadstrain: 0%, testing time: 168 hours) in which the immersion of the longtime was conducted without applying any fixed-strain bend. As shown inthe photograph, IGC was observed.

It has been confirmed from these test results that IGC occurs when thetemperature of a solution is raised to 80° C. or the concentration of anaqueous solution of K₂S₄O₆ is raised to 10%.

In FIG. 7, Test No. 7 and Test No. 8 show the conditions of tests byconventional methods for the initiation and growth of IGSCC and theresults of the tests. For the initiation and growth of IGSCC, Test No. 7required as much as from 1,000 to 3,000 hours, and Test No. 8 isconsidered to require an extremely long time in excess of 200,000 hoursbased on a value estimated from laboratory data.

Under the solution conditions A of Tests Nos. 1 and 4, said solutionconditions A meeting the conditions of the present invention, nocracking occurs unless a stress strain is applied but cracking takesplace when a stress strain is applied. Therefore, as illustrated in FIG.16, the initiation and growth time of IGSCC are considered to havestrain-dependency, and are also considered to be explainable based onthe film-destructive cracking mechanism [in this respect, reference maybe had to Pugh: Environment-Sensitive Mechanical Behavior, page 351,Cordenand Breach, N.Y., (1966)].

Moreover, the cracking is of the IGSCC type because it grew by selectingonly grain boundaries. This is equivalent to IGSCC of a sensitizednickel-based alloy in oxygen-containing water obtained in Test No. 7 orTest No. 8 in FIG. 7. However, the composition and properties of a filmare considered to differ depending on the conditions for its formation.

From the results of the above study, the test conditions of the presentinvention were limited to the hatched area shown in FIG. 1. The lowerlimit of the concentration of the aqueous solution of K₂S₄O₆ has beenset at 0.3 wt. % in the conditions of the present invention because, asIGSCC occurs in 24 hours even at 1 wt. %, it is desired to make itpossible to use the method of the present invention even when a needarises to delay the initiation time of IGSCC in the case of ahigh-stress welded structure. The setting of the upper limit of theconcentration at 6 wt. % is attributed to the possibility of IGCinitiation if the concentration is set at a level as high as 10 wt. %.The concentration of K₂S₄O₆ can, therefore, be in a range of from 0.3 to6 wt. %, preferably from 0.5 to 3 wt. %.

The setting of the lower limit of the solution temperature at 5° C. isto avoid freezing of test solutions. The setting of the upper limit ofthe solution temperature at 60° C. is to inhibit the occurrence ofpitting corrosion or IGC. Accordingly, the solution temperature can bein a range of from 5 to 60° C., preferably from 10 to 30° C.

A metal area other than an area where the growth of IGSCC is intendedcan be covered with a non-metallic material such as a silicone rubber orfluororubber as needed. When covered as described above, IGSCC can becaused to initiate only at the partially exposed metal area by bringinga test solution into contact with the partially exposed metal area evenin the case of a large welded equipment structure.

Further, the initiation of IGSCC can be facilitated by forming at leasta portion of a welded structure with a material the stabilizationparameter of which is 12 or smaller, or by applying heat treatment to apartial specimen of a welded structure 600° C. and then low-temperatureaging to it at 288 to 550° C. such that the specimen is brought into asensitized state.

A description will next be made about a method according to anotherembodiment of the present invention for the initiation and growth ofintergranular stress corrosion cracking in a specimen. In thisembodiment, the selection of test conditions was conducted in furtherdetail by using an orthogonal table L₉(3⁴) in quality engineering.

The orthogonal table L₉(3⁴) is a sort of experimental design, whichmakes use of an orthogonal table. The “9” in Lg represents the number ofrows in the orthogonal table (this number corresponding to the number Nof experiments upon allocation), the “3” in 34 indicates that thenumbers which make up each column each consists of three numerals(corresponds to the 3 levels of each factor), and the “4” of 34indicates that the number of columns is 4 columns (corresponds to thenumber of the factors).

FIG. 17 summarizes the results of a fixed-strain bend test conducted byallocating, as SCC-affecting factors, K₂S₄O₆ concentration, pH value,temperature and NaCl concentration in the orthogonal table Lg.

The K₂S₄O₆ concentration was set at 1, 3 and 6 wt. %. The pH value wasset at 1, 3 and unadjusted condition in order to investigate its degreeof influence. In Tests Nos. 4 and 7 in each of which the pH value wasnot adjusted as a condition, the pH values fell below 4. This isconsidered to be attributable to the high K₂S₄O₆ concentrations. Thetest temperature was set at 25, 40 and 60° C. for detailed observation.The addition of NaCl was effected on a trial basis to investigate anypossible effect for shortening the time of an SCC test, and the contentof NaCl was set at non-addition, 0.1 wt. % and 1 wt. %.

In the fixed-strain bend test, two test pieces were used for each testcondition, and a 24-hour immersion test was conducted. To compare thereadiness of cracking in the respective tests, the test results weresummarized as shown in FIG. 17. Specifically, the total length andnumber of surface cracks of IGSCC occurred in the surface of each testpiece were determined, and the crack length per crack was thendetermined. Using the results, the SN ratio of large characteristics wasdetermined in accordance with the following formula, and the results arepresented in FIG. 17.${SN} = {{- 10}\quad\log\left\{ {\frac{1}{n}\quad{\sum\limits_{i = 1}^{n}\left( \frac{1}{y_{i}} \right)^{2}}} \right\}}$where n stands for the number of data and y_(i) is a characteristicvalue.

FIGS. 18A to 18D show factorial effects based on the SN ratios. FIG. 18Ais a table showing variations in SN ratio depending on the K₂S₄O₆concentration, FIG. 18B is a table showing variations in SN ratiodepending on the pH value, FIG. 18C is a table showing variations in SNratio depending on the temperature, and FIG. 18D is a table showingvariations in SN ratio depending on the NaCl concentration.

As apparent from these tables, the SN ratio is higher and cracking iseasier when the K₂S₄O₆ concentration is 3 wt. % or 6 wt. % than 1 wt. %.Concerning the pH value, cracking is relatively easier at pH 3, butcracking becomes difficult when the pH value is excessively loweredto 1. At low pH values, test pieces and a fixed-strain bend test jigmade of SUS 304 show appearances of uniform or general corrosion andbecome black. Slower cracking at lowered pH values may possibly berelated to this phenomenon. With respect to the temperature, cracking iseasier at 40° C. or 60° C. than at 25° C.

When NaCl is added to the concentration of 0.1 wt. %, IGSSC isaccelerated than that by the same solution without addition of NaCl.This can be appreciated well if FIG. 18D is converted into a graphicdiagram as shown in FIG. 19. Accordingly, the upper limit of the NaClconcentration is 10 wt. %. As the molecular weight of NaCl is 58.5 andthe atomic weight of Cl is 35.5, the 10 wt. % concentration of NaCl canbe converted in to Cl concentration as follows: 10×(35.5/58.5)=6, thatis, 6 wt. %, while the 0.1 wt. % concentration of NaCl can be convertedto 0.06 wt. % in terms of Cl concentration. In terms of Clconcentration, the NaCl concentration can therefore be in a range offrom 0.06 to 6 wt. %, preferably in a range of from 0.6 (which isequivalent to 1 wt. % in terms of the NaCl concentration) to 6 wt. %.KCl and the like are also considered to accelerate SCC owing to thefilm-deteriorating effect of chlorine, a similar test was conducted withKCl. As a result, it has also been found that the NaCl concentration canbe in a range of from 0.06 to 6 wt. %, preferably in a range of from 0.6to 6 wt. % in terms of the Cl concentration.

Because a reduction in the pH value of the solution makes it possible toinitiate not only IGSCC but also IGS, The lower limit of the pH value isset at 3. To raise the pH value, it is necessary to add another chemicalin a large amount. This is inconvenient from the standpoint of handlingso that the upper limit of the pH value is set at 6. As shown in FIG.49, there is no substantial difference in S/N between the case of pH 4.5and the case of pH 6, and the S/N at pH 4.5 is substantially equal tothat at pH 6. Accordingly, the pH value of the solution can be from 3 to6, preferably from 4.5 to 5.5.

From the results of the above study, the conditions suitable for theinitiation of IGSCC in Alloy 182 were selectively determined as will bedescribed hereinafter. When the concentration of K₂S₄O₆ is raised, thepH value is lowered to initiate IGC. The concentration of K₂S₄O₆ was,therefore, set at from 0.3 to 6 wt. %. The pH value was set at from 3 to6 to avoid any initiation of IGC. The temperature was set at from 5 to60° C. by also taking handling ease into consideration. The chlorineconcentration was limited to a range of from 0.06 to 6 wt. % with a viewto accelerating SCC.

FIG. 20 illustrates the bend-strain-dependency of sensitized Alloy 182in the solution 1 (an aqueous solution of 1% K₂S₂O₆) and the solution 2(an aqueous solution of 0.1% NaCl+1% K₂S₂O₆) as determined by an OBBtest. As apparent from the graphic diagram, the service life until theinitiation of IGSCC in the solution 2 was as short as one hundredth ofthe service life until the initiation of IGSCC in the solution 1. It is,therefore, understood that the service life until the initiation ofcracking in Alloy 182 is considerably shortened by the addition of 0.1%NaCl (equivalent to 0.6 wt. % in terms of Cl concentration).

FIG. 21 is a perspective view for describing the shape and material of aspecimen according to an embodiment of the present invention. Thespecimen 10 was prepared by cutting a V-groove in an approximatelycentral part of a member 11, which was 91 mm in thickness and was madeof Alloy 600, and welding the member with Alloy 182. The drawing shows aV-groove weld 12 and a single-pass weld 13. This specimen 10 wassubjected to heat treatment at 600° C. for 24 hours and low-temperatureaging at 500° C. for 24 hours. The arrangement of the single-pass weld13 (a single-pass weld bead of Alloy 182) in the specimen 10 of FIG. 16was to produce a welding residual stress under the welding conditionswithout any modification thereto. The single-pass Alloy 182 was formedfrom a welding consumable the stabilization parameter of which was notgreater than 8, and was in a sensitized state after the welding withoutneeding any further treatment.

FIG. 22 is a view showing a specimen 10 with a silicone rubber coating 3applied to an area other than an area where initiation of IGSCC wasintended. This specimen allows to specifically limit an area where IGSCCis to be initiated.

FIG. 23 is a table showing two kinds of test conditions and the testresults. In these two kinds of tests, the test solutions both met theconditions according to the present invention. Test No. M1 was a144-hour test conducted after the surface of the specimen was ground bya grinder, while Test No. M2 was a 168-hour test after the surface ofthe specimen was buffed. The grinding and buffing were conducted toinvestigate how they would affect the welding residual stresses whichinherently existed in the specimens. As a result of the tests, IGSCCwere found to occur in both of the tests.

FIG. 24 is a view depicting a test arrangement in which a weldedspecimen 10 was tested in a container 14 made of polytetrafluoroethylene[tradename: TEFLON (trademark)]. It was a liquid penetrant testconducted by placing the specimen 10 in the container 14, pouring a (1wt. %) aqueous solution 15 of K₂S₄O₆, and immersing the specimen 10 fora predetermined time in the aqueous solution 15.

FIG. 25 shows photographs, which present the results of the penetranttest of the specimen after the immersion test (see FIG. 24) conductedunder the conditions of Test No. 2 shown in FIG. 23. FIG. 25 shows thepositions of cracks in the circled areas, and also shows the enlargeddetails of the cracks.

FIG. 26 is a photograph, which confirmed the detection of cracks on theside of a cross-section (a side wall) of the V-groove weld 12 after thespecimen was washed and etched subsequent to the liquid penetrant test.One of the cracks so occurred was about 5 m in length (in a lower partof the V-groove weld), and the other was about 10 mm in length (in anupper part of the V-groove weld). FIG. 27 is a photograph of the IGSCCobserved on a fracture surface, and confirms that the cracks observed inFIG. 25 are IGSCC.

As apparent from FIG. 25, no cracks initiated in the single-pass weldbead of Alloy 182. It has hence been found that, even when thestabilization parameter is not greater than 12, cracking hardly takesplace if stress is small. It has also been found that IGSCC does notinitiate at all under the welding conditions of the present invention.

FIG. 28 shows views of further welded specimens {circle over (1)} and{circle over (2)}, each of which was prepared by welding a member 11 ofAlloy 600 and a member 16 of an austenite stainless steel together witha weld 12 of Alloy 182. These welds 12 of Alloy 182 become target areasfor the initiation and growth of IGSCC. IGSCC can be caused to initiateby exposing these specimens 10 to a solution of the present invention ina container as shown in FIG. 24 either as are or after covering themwith a non-metallic material such as a silicone rubber at areas otherthan the target areas for the examination of IGSCC initiation.

A still further, simulated specimen 10 depicted in FIG. 29 was preparedby forming a cladding weld overlay 18 of Stainless Steel 308 onlow-alloy steel 17 and welding a nickel-based alloy (Alloy 600) 19 onthe cladding weld overlay via welds 12 of Alloy 182. As the specimen 10is large and is made of the low-alloy steel 17 in this case, a dam isformed with TEFLON-made, plate-shaped solution-holding members 20 and asealing material of silicone rubber around the welds 12 of Alloy 182,which are the target areas for the initiation and growth of IGSCC, asillustrated in FIG. 30, and a test solution is then poured into the damto immerse the welds 12 of Alloy 182.

FIG. 30 is a view illustrating a modification of FIG. 31. In thismodification, a cylindrical solution-holding member 20 is used tosurround the circumference of a weld 12 of Alloy 182. If necessary,external force can be applied to the specimen to facilitate theinitiation of stress corrosion cracking.

Aqueous solutions of K₂S₄O₆ were used as test solutions in theabove-described embodiments. The present invention is, however, notlimited to them, and solutions of other tetrathionate salts, forexample, aqueous solutions of Na₂S₄O₆ are also usable.

The technique of the present invention has made it possible tospecifically limit an initiation area of IGSCC, and therefore, is veryuseful in technologically improving, for example, the equipment andmethods for ultrasonic examination tests and eddy-current examinationtests. As a method for specifically limiting an initiation area ofIGSCC, it has already been mentioned to cover an area other than theinitiation area with a non-metallic material such as rubber or resin. Itis, however, necessary to cover a wide area with a non-metallic materialwhen an area where IGSCC may initiate is broad although IGSCC is desiredto initiate in only an extremely small part of the area.

A description will next be made about a testing method which allows auser of a specimen to simply and easily cause IGSCC to initiate at adesired position in a manner other than the above-described coating.Although it is effective to specifically limit an initiation area ofIGSCC by coating, there is a potential problem that a flow of ions orthe like indispensable for the initiation of SCC may be blocked by thecoating. It is, therefore, difficult to specifically limit an area,which is to be brought into contact with a test solution, to a verysmall area. In contrast to the above-described method, the testingmethod to be described next is free of such a potential problem, allowsIGSCC to initiate and grow well, and also, makes it possible tospecifically limit an area, which is to be brought into contact with atest solution, to a very small area.

This method is characterized in that after preparation of a weldedstructure, in other words, after welding a structure, the structure ismechanically ground or polished by a grinder or buff at the weld beadonly in an area where the initiation of IGSCC is desired and the weldbead in the remaining area is left unground or unpolished, and in thisstate, the structure is immersed in a K₂S₄O₆ solution to cause IGSCC toinitiate and grow. The area where the surface has been ground by thegrinder or the like serves as an area having a high potential for theinitiation of IGSCC, thereby making it possible to cause IGSCC topreferentially initiate at the specific area.

FIG. 32 is a flow chart of a first testing method. In S1, a plate-shapedspecimen made of a metal capable of retaining corrosion resistancethrough passivation, for example, a nickel-based alloy or the like and awelding consumable made, for example, of a nickel-based alloy areprovided (S1).

The specimen is mounted on a welding deformation restraint plate orrestraint jig (S2), and is welded in the mounted state (S3). Subsequentto the welding, a bead is ground and finished by a grinder or the likeonly at an area where the initiation of IGSCC is desired in S4. In S5,the specimen is set on a deformation applying jig the upper surface ofwhich is bulged in a slightly curved form as shown in FIG. 6B, and istightened by bolts and nuts to apply a mechanical deformation thereto.Although the mechanical deformation was applied by bending in thisembodiment, the mechanical deformation can also be applied in adifferent manner, for example, by pulling or the like.

Subsequently, the specimen is immersed for a predetermined time in anaqueous solution of K₂S₄O₆ (S6). After the specimen is pulled out of thesolution and the initiation and growth of IGSCC are confirmed (S7), thespecimen is ground and finished by a grinder or the like at the areaother than the area where IGSCC has initiated (S8).

In this testing method, the welding can be directly conducted byomitting the mounting of the specimen on the welding deformationrestraint plate or restraint jig in S2. Further, the specimen can bedirectly immersed in the aqueous solution of K₂S₄O₆ by omitting theapplication of the mechanical deformation to the specimen in S5.

FIGS. 33A through 33D are perspective views, which specificallyillustrate the testing method making use of the grinding means. Asdepicted in FIG. 33A, a simulated support plate 22 made of Alloy 600 isbrought at a free end thereof into abutment against a side wall of asimulated support cylinder 21 made of Alloy 600, and as illustrated inFIG. 33B, they are welded together. One of the thus-formed weld beads 23is ground by a grinder at an area where IGSCC is intended to initiate,so that a ground part 24 is formed to provide a specimen.

This specimen is immersed for a predetermined time in theabove-mentioned test solution such that IGSCC 25 is caused to initiateand grow as shown in FIG. 33C. After confirming it, the weld bead 23 isground and finished in its entirety by a grinder as shown in FIG. 33D.

FIG. 34 is a flowchart of a second testing method. This flow chart isdifferent from the flow chart shown in FIG. 32 in that, after theinitiation and growth of IGSCC have been confirmed in S7, the weld beadis ground and finished at the remaining area thereof by a grinder or thelike and is immersed in the aqueous solution of K₂S₄O₆ to cause newIGSCC to initiate and grow in S7-1 and the new IGSCC is confirmed inS7-2. By this method, IGSCC can be caused to initiate and grow on asingle specimen at plural areas with a time interval.

FIG. 35 is a flow chart of a third testing method. This flow chart isdifferent from the flow chart shown in FIG. 32 in that, after welding inS3, the weld bead is ground and finished at plural areas thereof by agrinder or the like and is partially coated with a non-metallic materialin S4-1 and also in that, after the initiation and growth of the firstIGSCC have been confirmed in S7, the above-mentioned coating is removedand the specimen is immersed again in the aqueous solution of K₂S₄O₆ tocause new IGSCC to initiate and grow and the new IGSCC is confirmed inS-7. By this method, IGSCCs can be caused to initiate and grow withdifferent contact time periods with the aqueous solution of K₂S₄O₆.

FIG. 36 is a photograph of a specimen. The photograph shows areas A andan area B of a weld bead, which were grinder-finished andgrinder-unfinished, respectively, after Alloy 600 (portions indicated byletter E in the figure) was welded with Alloy 182 (a portion indicatedby letter D in the figure).

FIG. 37 is a photograph showing that patterns C were detected at thegrinder-finished areas A by immersing the specimen in the aqueoussolution of K₂S₄O₆ and conducting a penetrant examination after theimmersion. It has been found from the detection that IGSCCpreferentially initiates and grows at a grinder-finished area.

Because Alloy 600 having good corrosion resistance was used as a basemetal in the specimen, IGSCC did not initiate in Alloy 600 under theabove-described immersion conditions.

A description will next be made about sensitization treatment of aspecimen. FIG. 38 is a simplified view showing an illustrative weldedjoint (welded structure) in an in-core structure of a pressure vessel ina nuclear power plant. The figure shows stainless steel base metals 31joined together via a weld metal 32 of ferrite-containing austenitestainless steel. To perform maintenance against SCC in a weld of such astructure, a technique is needed to cause IGSCC to initiate and grow onspecimens.

FIG. 39 is a table showing an illustrative chemical composition of theweld metal 32 of ferrite-containing austenite stainless steel. In thecase of this material, the content of ferrite is 8 vol. %. Those havinga ferrite content of from 8 to 15 vol. % are generally used. FIG. 40 isa table showing illustrative welding conditions (welding rod diameter,current, voltage, welding speed, heat input, interlayer temperature)upon preparing a specimen.

FIG. 41 is a diagram illustrating how to cut out, from the weldedstructure shown in FIG. 38, a cut-out specimen 33 with the weld metal 32of ferrite-containing austenite stainless steel included therein. Thiscutout specimen 33 has a V-groove weld in a substantially central partthereof.

To such specimens 33, heat treatment was applied at 610° C. for 6 hoursto 40 hours, respectively, as illustrated in FIG. 42. On the specimenssubjected to the heat treatment, a modified ASTM A262E test shown inFIG. 43 was conducted. Test conditions were as shown in the same figure.

FIG. 44 is a simplified construction diagram of the test apparatus. Asillustrated in the diagram, a test solution 35 (16% H₂SO_(4+6.4)% CuSO₄)for the modified ASTM A262 test is placed in a test container 34, theabove-mentioned specimen 33 is immersed in the test solution 35, andthen, the test solution 35 is heated and boiled by a heater 36.Generating steam is condensed into dew with cooling water 38 fed througha condenser tube 37, and water droplets are allowed to return to thetest container 34. In the modified ASTM A262E test, it is stipulated toconduct the test with a copper piece 39 kept in contact with thespecimen 33.

FIG. 45 is a photograph showing a cross-section of the specimen 33 afterthe modified ASTM A262E test of the weld metal of ferrite-containingaustenite stainless steel, which had been subjected to heat treatment at610° C. for 6 hours (the heat treatment conditions No. 1 in FIG. 42),was conducted. It is understood from this photograph that the specimen33 had developed intergranular corrosion.

FIG. 46 is a characteristic diagram showing the behavior ofintergranular corrosion when heat treatment was applied to a weld metalof ferrite-containing austenite stainless steel at 610° C. for differentperiods in a range of from 6 hours to 40 hours. It is appreciated fromthe diagram that by applying heat treatment to a specimen at 610° C. for6 hours to 15 hours, substantial intergranular corrosion takes place tosensitize the specimen.

From the foregoing, it has been found that a specimen is sensitized whena weld metal of ferrite-containing austenite stainless steel issubjected to heat treatment at 610° C. for 6 hours to 15 hours. It was,therefore, investigated how the heat treatment time would vary in atemperature range of from 500 to 650° C. by making use of the activationenergy for Cr diffusion which plays a role in the sensitization. Theresults of the investigation will be described hereinafter.

Heat treatment is applied to a specimen within the following temperaturerange:

-   -   500° C.≦T° C.≦650° C.        and the following time range:    -   t₁ hours≦t hours≦t₂ hours        where $\begin{matrix}        {t_{1} = {6\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\        {t_{2} = {15\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\        {{Q = {50,000\quad{to}\quad 60,000\quad{cal}\text{/}{mol}}},{and}} \\        {R = {1.987\quad{cal}\quad K^{- 1}{{mol}^{- 1}.}}}        \end{matrix}$

By this heat treatment to the specimen, Cr-depleted layers are formed atboundaries between ferrite phases and austenite phases as a result ofthe deposition of Cr carbides so that the specimen can be sensitized.

The lower limit of the treatment temperature was set at 500° C., becausethe heat treatment time ranges from 346 to 865 hours when Q=50,000cal/mol and the heat treatment time becomes excessively long attemperatures lower than 500° C. On the other hand, the upper limit ofthe treatment temperature was set at 650° C., because at temperatureshigher than the lower limit, the concentration of Cr in the interfacesof Cr carbides becomes so high that sensitization can be hardlyachieved. The activation energy Q for sensitization was set at 50,000 to60,000 cal/mol by determining it from a range which the activationenergy for Cr diffusion can take.

By bringing a specimen, which has been sensitized as described above,into contact with a K₂S₂O₆ test solution or a NaCl+K₂S₂O₆ test solution,IGSCC initiates well and the state of its growth can be observed.

FIG. 47 is a perspective view showing an arrangement for the test. On aweld metal 32 of ferrite-containing austenite stainless steel, acontainer 40 which is open at the bottom thereof and is made of TEFLON,for example, is fixedly arranged with a silicone rubber 41. A testsolution 42 consisting of the aqueous solution of K₂S₄O₆ or the aqueoussolution of NaCl+K₂S₄O₆ was poured into the container 40 so that IGSCCis caused to initiate and grow in the weld metal 32.

FIG. 48 is a simplified view showing the state of an IGSCC 43 initiatedand grown in the weld metal 32 of ferrite-containing austenite stainlesssteel.

The technique according to the present invention for the initiation andgrowth of IGSCC is useful in a variety of technical fields, for example,for purposes such as:

-   -   (1) improvements in UT technology,    -   (2) development of inspection techniques other than UT,    -   (3) development of repair techniques for Inconel structures,    -   (4) verification of reasonability of the results of a residual        stress analysis of a welded structure by the elastoplastic        analysis,    -   (5) empirical verification of an examination technique for        potential SCC in a mockup specimen of a welded structure in        actual equipment, and    -   (6) various surface treatment confirmation tests.

Industrial Applicability

The first aspect of the present invention is characterized by bringing aspecimen, which is made of a material capable of retaining corrosionresistance through passivation, into contact with a solution of atetrathionate salt such that intergranular stress corrosion crackinginitiates and grows in the specimen. It is, therefore, possible tosimply and easily cause only IGSCC to initiate at atmospheric pressurein a short period.

The second aspect of the present invention is characterized in that thespecimen is a welded structure or a simulated specimen of the weldedstructure, or a cut-out specimen cut out from the welded structure orthe simulated specimen. It is, therefore, possible to determine theposition and extent of initiated IGSCC in a short time in a laboratoryunder the conditions of the material of a structure in actual equipmentor under welding residual stress in the structure.

The third aspect of the present invention is characterized by providingmeans for applying a strain to the specimen such that the specimen isbrought into contact with the tetrathionate salt solution while beingapplied with a strain by the means. The fourth aspect of the presentinvention is characterized in that the means for applying a strain tothe specimen is a weld applied to the specimen. Further, the fifthaspect of the present invention is characterized in that the means forapplying a strain to the specimen is a member or device for applying astrain to the specimen from an outside. It is, therefore, possible toprecisely determine the initiation and state of growth of IGSCC.

The sixth aspect of the present invention is characterized in that thesolution of the tetrathionate salt is an aqueous solution of potassiumtetrathionate or sodium tetrathionate, and a concentrate of potassiumtetrathionate or sodium tetrathionate, a temperature of the solution anda pH value of the solution are controlled to a range of from 0.3 to 6wt. %, a range of from 5 to 60° C. and a range of from 3 to 6,respectively. It is, therefore, possible to cause only IGSSC to surelyinitiate in the specimen.

The seventh aspect of the present invention is characterized in that theaqueous solution of potassium tetrathionate or sodium tetrathionatecontains chlorine in a range of from 0.06 to 6 wt. %. Further, theeighth aspect of the present invention is characterized in aconcentration of the chlorine is controlled to a range of from 0.6 to 6wt. %. It is, therefore, possible to cause only IGSCC to more surelyinitiate in the specimen.

The ninth aspect of the present invention is characterized by coveringthe specimen with a non-metallic material at an area other than an areawhere initiation of the intergranular stress corrosion cracking isintended. Further, the tenth aspect of the present invention ischaracterized in that the non-metallic material is a silicone rubber orfluororubber. It is, therefore, possible to specifically limit an areaof the specimen for the initiation of IGSCC as desired.

The eleventh aspect of the present invention is characterized in thatthe specimen has a weld, and the weld is composed of a nickel-base alloyor stainless steel. It is, therefore, possible to more surely cause onlyIGSCC to initiate on the specimen.

The twelfth aspect of the present invention facilitates the initiationand growth of IGSCC by controlling the stabilization parameter of thespecimen to 12 or smaller.

The thirteenth aspect of the present invention applies heat treatment tothe specimen to bring the specimen into a sensitized state beforecausing the intergranular stress corrosion cracking to initiate in thespecimen. Therefore, the initiation and growth of IGSCC are facilitated.

The fourteenth aspect of the present invention specifies the temperaturerange and time range of heat treatment to the specimen. Accordingly, theinitiation and growth of IGSCC are facilitated further.

The fifteenth aspect of the present invention is characterized in thatthe specimen has a weld, and the weld is composed of aferrite-containing austenite stainless steel. It is, therefore, possibleto more surely cause only IGSCC to initiate in the specimen.

The sixteenth aspect of the present invention is characterized in thatthe specimen has a weld, a surface of a weld bead of the weld is groundat an area thereof where initiation of intergranular stress corrosioncracking is intended, and the solution of the tetrathionate salt isbrought into contact with the thus-ground surface. It is, therefore,possible to specifically limit an IGSCC initiation area of the specimenas desired, and moreover, to cause IGSCC to initiate even when thespecifically-limited area is very small.

The seventeenth aspect of the present invention is characterized in thata dam is formed on the specimen in a vicinity of an area whereinitiation of intergranular stress corrosion cracking is intended, andthe solution of the tetrathionate salt is poured into the dam to causethe intergranular stress corrosion cracking to initiate and grow in oraround the weld. It is, therefore, possible to conveniently apply thetest to large equipment such as actual equipment.

The eighteenth aspect of the present invention is characterized in thatthe specimen is a welded structure in a nuclear power plant or asimulated specimen of the welded structure, or a cut-out specimen cutout from the welded structure or the simulated specimen. It is,therefore, possible to determine the soundness or the like of materialsof equipment such as reactors, in-core structures and weld zones innuclear power plants.

1. A method for causing intergranular stress corrosion cracking toinitiate and grow in a specimen made of a material capable of retainingcorrosion resistance through passivation, which comprises bringing saidspecimen into contact with a solution of a tetrathionate salt such thatsaid intergranular stress corrosion cracking initiates and grows in saidspecimen.
 2. A method according to claim 1, wherein said specimen is awelded structure or a simulated specimen of said welded structure, or acut-out specimen cut out from said welded structure or said simulatedspecimen.
 3. A method according to claim 1 or 2, further comprisingproviding means for applying a strain to said specimen such that saidspecimen is brought into contact with said tetrathionate salt solutionwhile being applied with a strain by said means.
 4. A method accordingto claim 3, wherein said means for applying a strain to said specimen isa weld applied to said specimen.
 5. A method according to claim 3,wherein said means for applying a strain to said specimen is a member ordevice for applying a strain to said specimen from an outside.
 6. Amethod according to any one of claims 1-5, wherein said solution of saidtetrathionate salt is an aqueous solution of potassium tetrathionate orsodium tetrathionate, and a concentrate of potassium tetrathionate orsodium tetrathionate, a temperature of said solution and a pH value ofsaid solution are controlled to a range of from 0.3 to 6 wt. %, a rangeof from 5 to 60° C. and a range of from 3 to 6, respectively.
 7. Amethod according to claim 6, wherein said aqueous solution of potassiumtetrathionate or sodium tetrathionate contains chlorine in a range offrom 0.06 to 6 wt. %.
 8. A method according to claim 7, wherein aconcentration of said chlorine is controlled to a range of from 0.6 to 6wt. %.
 9. A method according to anyone of claims 1-8, further comprisingcovering said specimen with a non-metallic material at an area otherthan an area where initiation of said intergranular stress corrosioncracking is intended.
 10. A method according to claim 9, wherein saidnon-metallic material is a silicone rubber or fluororubber.
 11. A methodaccording to anyone of claims 1-10, wherein said specimen has a weld,and said weld is composed of a nickel-base alloy or stainless steel. 12.A method according to anyone of claims 1-11, wherein said specimencontains C, Nb and Ti, and a stabilization parameter of a materialmaking up said specimen as determined by the following formula is 12 orsmaller:Stabilization parameter=0.13×[(Nb+2Ti)/C] where C, Nb and Ti indicatecontents of the respective constituent materials as expressed in termsof wt. %.
 13. A method according to anyone of claims 1-12, furthercomprising applying heat treatment to said specimen to bring saidspecimen into a sensitized state before causing said intergranularstress corrosion cracking to initiate in said specimen.
 14. A methodaccording to claim 13, wherein said heat treatment is applied to saidspecimen within the following temperature range: 500° C.≦T° C.≦650° C.and the following time range: t₁ hours≦t hours≦t₂ hours where$\begin{matrix}{t_{1} = {6\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\{t_{2} = {15\quad\exp\left\{ {\frac{Q}{R}\left( {\frac{1}{T + 273} - \frac{1}{883}} \right)} \right\}}} \\{{Q = {50,000\quad{to}\quad 60,000\quad{cal}\text{/}{mol}}},{and}} \\{R = {1.987\quad{cal}\quad K^{- 1}{{mol}^{- 1}.}}}\end{matrix}$
 15. A method according to claim 13 or 14, wherein saidspecimen has a weld, and said weld is composed of a ferrite-containingaustenite stainless steel.
 16. A method according to anyone of claims1-8 and 11-15, wherein said specimen has a weld, a surface of a weldbead of said weld is ground at an area thereof where initiation ofintergranular stress corrosion cracking is intended, and said solutionof said tetrathionate salt is brought into contact with the thus-groundsurface.
 17. A method according to anyone of claims 1-16, wherein a damis formed on said specimen in a vicinity of an area where initiation ofintergranular stress corrosion cracking is intended, and said solutionof said tetrathionate salt is poured into said dam to cause saidintergranular stress corrosion cracking to initiate and grow in oraround said weld.
 18. A method according to any one of claims 1-17,wherein said specimen is a welded structure in a nuclear power plant ora simulated specimen of said welded structure, or a cut-out specimen cutout from said welded structure or said simulated specimen.